I approve of SPECIAL-VARIABLE-TEST:SPECIALP (except why isn't
the proposal named SPECIAL-VARIABLE-TEST:SPECIAL-VARIABLE-P ?)
For the current practice section: Symbolics Genera has a function
named SYS:SPECIAL-VARIABLE-P that does this. The function is not
documented, currently, probably because we've been waiting to see
if Common Lisp standardizes on a different name.